Electric power distribution management by electric utilities in many countries is affected by a matter that involves the values of the total losses of electric power, comprised by portions known as “commercial losses” and “technical losses”, which present values well above the international average and acceptable values for the kind of service being paid properly, causing economic losses to society as a whole.
“Technical losses” originate from the passage of electric current through equipment and distribution networks, such as Joule effect losses in the conductors, losses in Watts in the transformer and reactor cores, in the capacitor banks, etc, being inherent in any system of transmission and distribution of electric energy.
“Commercial (non-technical) losses” are created by fraud in measurement systems, power diversion before measurement, illegal connections, billing errors in processing, defective measuring equipment, and registration problems, among others.
In order to combat and reduce these losses, utilities have made use of various technologies and work processes, often involving high investment and operating costs without adequate return. Due to its greater complexity, these solutions also have installation and maintenance costs much higher than those incurred by conventional connections. Additionally, as a regulatory point of view, these losses have not been totally incorporated in the tariff revisions and readjustments, many utilities have difficulty in making greater investments to step up the battle against power losses, characterizing a vicious circle often difficult to break to achieve a better management program and reduce the losses.
A major difficulty faced in monitoring and identifying possible actions designed to cause underinvoicing of real consumption of electric power consumers is the broad range of electrical load variation, characterized in that the electric current varies practically from zero to the current capacity limit of the circuit to which it is connected. For example, any reduction in the current value may have been produced by an action seeking to cause underbilling of consumption, due to failure or defect in the measurement system or said loads may truly have been disconnected. This type of action, which acts on the electric current quantity in the electric power facilities where the consumer units are measured, is a frequent cause of commercial losses of electric utilities, either by diversions in the branch connection (before measurement by the utility) or at the billing measurement system facilities.
The consumption units or substations that have high installed electric power make use of transformers for billing or operating measurement instruments, either for use with potential transformers and current transformers together, or for use with current transformers only. These are traditionally known as indirect measurement facilities.
Currently, the current transformers installed for operational or billing measurement of consumption units or substations having indirect measurement do not have any internal monitoring information that allows the effective and on-going verification whether the current signal that feeds external electric power meters or other instruments arranged to measure the consumption at the substation or consumption unit is being adequately transferred.
From the secondary windings of the current transformers to the inside of electric power meters, various irregularities by way of frauds may occur, such that the electric power values recorded by these meters are lower than those genuinely consumed by a consumption unit. These frauds, applied to the current signal, might short-circuit the secondary conductors that connect the secondary windings of said current transformers to the electric power meters, insert bypasses in the current circuit coming from the test switches, short-circuit the coils or current circuits inside the electric power meters, etc. Moreover, it is important to point out that these actions can be intermittent or temporary, and in some situations not even an inspection is able to determine that frauds are being used, since they are withdrawn prior to inspection. These are often put back in place when the utility's teams leave the consumption unit site after an inspection. These actions are designed to cover up an irregularity which masks a reduction in consumption.
The reduction in the value of electric current measured that circulates in the current transformer may be caused by simple load disconnection, by connection error (on purpose or not) in the current transformers or in the respective secondary conductors, or by the use of frauds or irregularities, so as to cause underinvoicing in the consumption of electric power real recorded by the meter that measures the consumption at the substation or consumption unit, sometimes making it difficult to prove, even by legal actions, that wrongful acts have taken place, chiefly in the case of temporary irregularities.
The most modern electronic meters of electric power already known in the state of the art have mass storage and are able to present a load curve and detect idle currents, as disclosed, for example, in North American patent U.S. Pat. No. 5,924,051, owned by General Electric Company, related to a meter's capacity record load curves. However, the simple confirmation of an idle current is not sufficient to prove fraud, since it could be caused by a load reduction.
Some techniques are designed to implement an inspection by way of instantaneous current meters, such as, for example, that described in the utility model application number MU 8303368-8 U which describes a system for comparing instantaneous currents and real time communication to assess diversions or irregularities in the distribution facility. Said application is based on detecting diversions of instantaneous currents and does not use the accumulated current for this function, causing the need for digital storage memory for storing information of instantaneous currents or the use of a communication system between the stations in order to make real time comparison.
There are also techniques that use the already widely known principle of measuring the difference in current between the secondary windings of two current transformers, as described in patent application PI0505840-6 A, to indicate deviations of current. Nevertheless, such technique, besides not being applied to cases of indirect measurement of billing, does not quantify consumptions of the Ampere-hour (Ah) quantity, which is equivalent to the time-integrated electric current, does not monitor potential failures in the current transformers, needs a dedicated power source, is not capable of identifying current diversions inside the electric power meters and also does not have the characteristics of inviolability of the present invention. This document also fails to identify tampering in the measurement system caused by wrongful acts in circuits for potential measurement, as embodied by the present invention, by means of recording the Volt-hour (Vh) quantity.
Some techniques try to monitor secondary circuits of the current transformers by means of injection of signals and their respective reception, as described in patent GB 2424286 A, there being the need for an additional and specific detector to identify possible tampering, which makes this solution more complex, besides not guaranteeing the identification of diversions inside the current circuit of the electric power meter and also not permitting the appropriate quantification of the consumptions of Ampere-hour (Ah) quantity. The aforementioned patent does not identify tampering in the measurement system associated with wrongful acts on the potential circuits by means of recording the Vh quantity, limiting the field of application.
Additionally, any failures in the current transformer cause an undesirable error in the transformation ratio. For example, a short-circuit between coils in the secondary current may make the nominal primary current/secondary current ratio decrease. If on-going monitoring is in place, this defect can be detected more quickly, with greater sensitivity and not just during periodical inspections.
Regarding both inductive and capacitive potential transformers, actions often occur aiming to provide under-recording of the real consumption measurement of the consumption unit. These actions may be the interruption of the secondary conductor that powers the potential circuit of the electric power meter to the insertion of voltage dividers in the respective circuits or even inside the meter, so as to cause underinvoicing of the real electric power consumed.
The most modern electronic meters known in the state of the art have the capacity to detect any voltage drops. Document WO9960415, assigned by ABB, for example, describes an apparatus and a method for detecting possible tampering in potential circuits by determining the displacement between the between the voltage distribution of the meter, but only applied in the case of measurement by means of two elements, three wires. In said patent, the voltage reductions applied to the meter, produced by unlawful means, are not also recorded in the potential transformers by means of the Volt-hour (Vh) meter.
Moreover, in many situations, the utility does not possess a record of voltages throughout the entire distribution and/or transmission system, and is unable to confirm, by comparison, whether the information on voltage drops stored in the meter are the result of unlawful actions, interruptions or faults in the distribution and/or transmission system or in the potential circuit of the meter, meaning it is hard to prove potentially unlawful acts.
An alternative found normally to minimize losses in indirect measurement installations consists of installing instruments outside the consumption units in the transformers, oftentimes encapsulated with the electronic power meters and accessory units for tele-measurement, in a single casing, as shown, for example, in document PI0402716, related to manufacturing processes of remote measurement sets. This solution often costs various times the value of a traditional measurement, besides requiring special groups for installation, maintenance, checking and field calibration, which are carried out in a live line in medium voltage distribution networks. The calibration service, for example, has to be performed in a public area, with the use of ladders and under different weather conditions, making this task, which is required by regulatory standards, complex, laborious and expensive.
Besides, this technique does not comprise permanent monitoring of the current or potential transformers already installed. This technique also presents as a difficulty and a disadvantage the fact that when failures occur in some of the assembly components, chiefly in the potential transformers and current transformers, or in cases of increase or reduction of contract load, the apparatus needs to be completely replaced, including all the costs as previously mentioned, besides additional complete sets for replacement stock.
It is also noted that current transformers do not have in their body or casing any means that shows information about the angular displacement (phase shift) between the secondary current and the primary current, when the transformers are in operation. Currently, this information is obtained by means of precision testing most often carried out in pre-set loading conditions of the current transformer and the respective secondary loads imposed upon its winding.
Accordingly, unless the current transformer is withdrawn for testing or by way of a complete field test procedure, it is not possible to make a real time evaluation of the performance of the current transformer considering the phase angle error to be a relevant aspect.
Therefore, the current technique consists of withdrawing the current transformer for laboratory testing of the angular displacement, and incurs costs of transporting, disassembling, reassembling, and also the risk of inappropriate handling. The precision curve of the current transformer can only be adjusted in the laboratory by programming the parameters in the electric power meter. In this case, the monitoring of the performance of the current transformers and of the eventual failures and unbalancing is still not carried out in real time.
Based on the above, it is concluded that in the current technique there are no devices or processes used in the transformer itself that are suitable for the current and potential transformers that allow information related to its internal performance in real time to be obtained. In addition, there are no known devices or processes applicable to current and potential transformers that perform the tasks mentioned previously and at the same time allow information related to the whole circuit to be obtained from the windings of the transformers to the very inside of the electronic or electro-mechanical meters, for verification, confirmation and legal proof of any irregularities or failures in the operating or billing measurement of the consumption units or substations that have this kind of equipment (indirect measurement).
According to the current state of the art, the inner software of certain electronic meters is able to identify certain events such as, for example, the current circuit reversal, absence of voltage in the potential circuit, opening the meter cover. Yet these resources have limited application, and are not sufficient to prove deviation, adulteration or any failures in the current and potential circuits of the indirect measurement associated to the respective instrument transformers. They also do not permit the immediate verification whether the reduction in consumption was real or the result of a fraud used to cause underbilling of the power consumption.